Biaxially stretched and blow-molded bottles made of polyethylene terephthalate (hereinafter referred to as “PET”), the so-called PET bottles, have high transparency, mechanical strength, heat resistance, and gas barrier property, and up to now, have been in wide use as the containers for various beverages. Conventionally, what is called hot filling is utilized as a method of filling the PET bottles with contents, e.g., juices, teas, and the like, which require pasteurization. This involves filling the bottle with the contents at a temperature of about 90 degrees C., sealing the bottle with a cap, and cooling the bottle. This process causes the pressure inside the bottle to decrease considerably.
As regards the application of use involving hot filling described above, Patent Document D1, for example, teaches that the body is provided with the so-called vacuum absorbing panels, which are, by design, easily deformed into a dented state under a reduced pressure condition. At the time of a decrease in pressure, these vacuum absorbing panels perform a vacuum absorbing function by deforming into the dented state, thus allowing the bottle to retain good appearance while ensuring that the portions of the bottle other than the vacuum absorbing panels have rigidity enough to avoid troubles on the bottle conveyor lines, during storage in piles, and inside the automatic vending machines.
On the other hand, in some cases it is necessary to avoid forming the vacuum absorbing panels on the body out of regard for the design of bottle appearance, or it is necessary for body walls to have high surface rigidity to give the body high retainability of shape enough to be able to stack the bottles on their sides inside the vending machines. For example, Patent Document D2 shows a synthetic resin bottle which has no vacuum absorbing panel in the body wall, but in which the vacuum absorbing function is performed by the upward drawing deformation of a bottom plate. Especially in the cases of small-size bottles with a capacity of 350 ml or 280 ml, the vacuum absorbing panels disposed in the body wall would have a limited panel area. In that case, it would be difficult to fully satisfy both of the vacuum-absorbing function and the rigidity or buckling strength of the body. Therefore, the vacuum-absorbing function need be performed by the deformation of bottom plate as described above.
As an example, FIG. 18 shows a bottle 101 in which the vacuum absorbing function is performed by a bottom plate of a bottom 105, which plate deforms so as to draw upward. FIG. 18(a) is a front view; and FIG. 18(b) is a bottom view. The bottle 101 comprises a body 104 having a thick wall and peripheral groove ribs 107 to give the body 104 high surface rigidity and high buckling strength. When there is a pressure drop inside the bottle, the body 104 retains its shape, but a sunken bottom portion 117 of the bottom 105 performs the vacuum absorbing function when this sunken bottom portion 117 deforms so as to draw further upward (i.e., deformation in an arrowed direction in FIG. 18(a)).